1. Field of the Invention
The present invention relates generally to piezoelectric ceramics, methods for making the piezoelectric ceramics, piezoelectric elements, liquid discharge heads, and ultrasonic motors. In particular it relates to a barium titanate piezoelectric ceramic that has controlled crystal grain size and thereby exhibits good piezoelectric performance and mechanical strength, a method for making the piezoelectric ceramic, a piezoelectric element that uses the piezoelectric ceramic, and a liquid discharge head and an ultrasonic motor that use the liquid discharge element.
2. Description of the Related Art
ABO3 perovskite oxides such as lead zirconate titanate (hereinafter referred to as “PZT”) are generally used as piezoelectric ceramics.
However, since PZT contains lead as the A-site element, PZT may have an adverse effect on the surrounding environment. Accordingly, piezoelectric ceramics that use lead-free perovskite oxides are sought after.
Barium titanate is known as a lead-free perovskite piezoelectric ceramic material. Japanese Patent Laid-Open No. 2008-150247 discloses barium titanate prepared by a resistance heating/two-step sintering technique. The '247 also discloses that when nano-sized barium titanate powder is sintered by the two-step technique, a ceramic that has a maximum grain diameter of 5 μm or less and good piezoelectric characteristics is obtained.
However, according to the two-step sintering technique, the retention time of the first sintering temperature needs to be short. As a result, the temperature of the ceramic to be sintered becomes inhomogeneous and the piezoelectric characteristics are not always satisfactorily reproduced. For example, the temperature of a barium titanate ceramic piece of a practical size does not become homogeneous if the retention time is about 1 minute. In other words, an ideal nanostructure is not realized in all parts of the sintered ceramics. Thus, there have been no barium titanate ceramics that achieve piezoelectric characteristics sufficient to replace PZT.
The piezoelectric performance of barium titanate may be improved by increasing the crystal grain diameter. Japanese Patent No. 4039029 discloses the relationship between the average grain diameter of a calcium-doped barium titanate ceramics and the piezoelectric constant. The piezoelectric constant (d31) is increased when the average grain diameter of the piezoelectric ceramic is increased from 1.3 μm to 60.9 μm.
In Japanese Patent No. 4039029, the average grain diameter of the ceramic is controlled by adjusting the time of wet-mixing of calcined powders. In addition, the average grain diameter of the ceramic is increased by increasing the primary firing temperature.
However, when the average grain diameter of the barium titanate ceramic is increased by the aforementioned techniques, the contact area between the crystal grains is decreased. Thus, the mechanical strength of the ceramic drops and ceramic parts become fragile during processing and driving of the piezoelectric element. In sum, it is desirable that barium titanate piezoelectric ceramics achieve both good piezoelectric characteristics and high mechanical strength.